The United States will step up enforcement of sanctions on Iran, national security adviser John Bolton said on Tuesday, as Tehran tries to find ways to evade the restrictions in oil trade and in banking.

“The objective has been from the beginning to get oil exports from Iran down to zero,” Bolton said.

“It is our intention to squeeze them very hard. As the British say: ‘Squeeze them until the pips squeak’.”

Eight importing countries have been given temporary exemptions by the United States to keep buying Iranian oil when it reimposed sanctions, prompting Iranian President Hassan Rouhani to say the Washington would not be able to cut Iran’s oil exports to zero.

Bolton also said the United States will let an investigation by Saudi Arabia’s public prosecutor to run through on the killing of journalist Jamal Khashoggi.

Khashoggi, a critic of Saudi Crown Prince Mohammed bin Salman, was killed last month at the kingdom’s Istanbul consulate by a team sent from Riyadh.

The kingdom’s public prosecutor Saud al-Mojeb later said Khashoggi was killed in a premeditated attack, but his body has not been found.

U.S. President Donald Trump remains prepared to hold a second summit with North Korean leader Kim Jong Un, Bolton said. The comments came after the release of a report detailing undeclared missile sites in the North that had been undergoing maintenance.

Bolton was speaking to reporters in Singapore on the sidelines of meetings this week between the ten-member Association of Southeast Asian Nations (ASEAN) and external partners, including the United States and China.